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Monday, December 12, 2016

Freezing Rain Hits Portland and Bellingham

On Thursday evening, while Seattle was worrying about approaching snow, Portland was being hit by freezing rain: liquid precipitation that freezes on contact with the ground or other objects. Extending from the western Columbia Gorge into the northern Willamette Valley, the Portland freezing rain lasted from 4 PM Thursday until noon on Friday, causing power outages and closing I 84 for hours.

Bellingham has also had several hours of freezing rain, mainly late Friday and Saturday, with a touch of it last night.

So why is the western Columbia Gorge, Portland, and the Bellingham region vulnerable to freezing rain, while it is an exceptional rarity here in Seattle? How can liquid water freeze on contact?

Freezing rain is associated with supercooled water. Strangely enough, water can be cooled to below freezing 0 C (32F) without freezing. This is not a rarity: much of the water in clouds is supercooled. To go from liquid water (where water molecules are moving around but are bound together) to ice (where water molecules are found in a rigid, crystalline structure) does not happen immediately when temperature drops to freezing. Sort of like a preschool class when you tell the kids to sit down....it takes a while. And having a template for the crystal structure from some kind of particle (such as a freezing nuclei) helps things along--and sometimes such particles are lacking in the free atmosphere.

Much of the liquid water in clouds is supercooled (below freezing).

Nearly all precipitation in the Northwest starts as snow aloft, but as it falls in our normally mild air (even in winter) it melts and turns to rain (see figure, left hand side). And if the atmosphere is cold through depth, the snow can reach the surface (right side). But in some situation, a shallow layer of below freezing air occurs near the surface, while warm air (above freezing) exists above. Thus, snow aloft can melt into rain and then get supercooled just above the surface. When it hits a cold surface it then freezes, thus freezing rain.

In the Northwest, a set up for freezing rain can occur in and downstream of gaps in our mountains. In winter, cold air is often found over eastern Oregon and Washington, with the coldest air prevented from reaching the west by the Cascades. Yes, some air can try to get over the mountains, but it rapidly warms as it sinks on the western slopes (as it is compressed by higher pressure). Only in and immediately downstream of sea-level gaps, such as the Columbia River Gorge or the Fraser Valley of southern BC can primo cold air get across. This is illustrated by the temperature forecast for 1 PM Thursday from the UW WRF system for 925 hPa (about 2500 ft). Cold temperatures are blue and white, warmer temperatures are yellow and light green.

On Thursday, there was cold, dense air east of the Cascades crest, associated with higher pressure. As a result, cold air accelerated westward down the Columbia Gorge, something shown by aircraft observations at Portland that day (see below). The figure provides a time-height cross section, with time increasing to the left (from 4 AM Thursday to 4 AM Friday) and temperature (red lines) and winds shown. Heights are in pressure (850 is about 5000 ft). Strong easterly winds are apparent at the surface, while the winds turned southerly/southwesterly aloft and the air warmed to just above freezing aloft.

With supercooled water falling on Portland, trees picked up lots of ice, causing branches to fall and power outages. And some of the roads became dangerously slick.

Bellingham has had a lot of cold gap flow the past few days as cold northeasterly flow surged out of the Fraser River Valley (which taps cold air in the interior of BC). This morning is no different (see surface map at 11 AM is shown). Below freezing in the northeasterly flow north of Bellingham, result in continued snow over NW Washington. Some of the NE flow is moving up the northern of the Olympics resulting in snow near Port Angeles. In contrast, south of Everett the temperatures are in the low 40s.

A view of the cam at the Peace Arch near the US/Canadian border looks white with fresh snow, and the situation on Hurricane Ridge on the northern side of the Olympic Mountains shows deep fresh snow. Northwest meteorologists must always be thinking of the gaps in our mountains when forecasting snow and freezing rain.

13 comments:

Freezing rain and drizzle are quite common east of the Cascades,too,as warm maritime air at times undercuts deep,cold air masses. We also had a brief bout of freezing drizzle late yesterday afternoon and evening in the Spokane area.

Well... we're maybe 50' of above sea level, a mile from Bellingham Bay, and 4" inches was on the ground at about 8am. Temps around 28.Raining hard last night, not sure when it turned to snow, but the 1/2" expected by NWS, was way off. Scraped the steps of snow between 8-9am, came back and another 1.5" had fallen. This 3rd undocumented event looked like snow to me, however since it was raining before changing over to white the road were a bit more slick under the white.

The Puget Sound was not without Freezing Rain on Friday either. I live on Kent's East Hill and from about 3pm to 11pm we were hit with freezing rain that stuck to the trees, plants, and elevated structures, but thankfully NOT the roads. My PWS showed temps below freezing (approx 31 degrees) but the radar showed rain. Accumulation was light, so most of the trees escaped damage (unlike the storm several years ago). Just to our east WSDOT had a hell of time with Hwy 18 due to snow. This is a perfect example of the illustration you provided, where the approaching front was warming the mid levels while the surface was still cold due to a strong outflow from eastern Washington thru Snoqualmie Pass.

Freezing rain might be a little more common across the flatter portion of Eastern Washington but in my short 30 yr life I've never seen a significant freezing rain event in the valleys east of the Cascades and for good reason. It's always sleet. Our deep and bowl like terrain traps the cold air thick enough to support it. So to get a melting layer several hundred or thousand feet up is difficult to achieve but it does happen. We currently have an icing event occurring now (Monday night) but it's due to heavy drizzle. Which people confuse with freezing rain.

I am near downtown Bellingham; I did not notice anywhere near as much freezing rain as described in Portland. I was out on my bike on Friday; did not have any trouble with ice on the roads and as far as I know, there were no power outages in inner north Bellingham. Likewise this morning (Monday), I took a hike in the snow from Cornwall Park area (where I live) to downtown and back and did not notice any ice on the trees or power lines.

The snow around Bellingham was incredibly localized this morning. The southern end of our street had a dusting, about 1/4 mile further north (on the same street) there was at least 1.5 inches. The north side of town received more than 4 inches. All within the space of about 3 or 4 miles.

And, to add onto what Unkown said, at 2:35 am, in Duvall, a tree branch got weighed down with too much frozen precip, knocking out power for a couple hours for 500 or so customers, one of them being my family's place. Wound up being the first customer to call PSE and report it.

This reminds me again of how much I've enjoyed reading your "Weather of the PNW," which I got for Christmas several years ago. Great book for casual reading, and a fine present for anyone who enjoys the out-of-doors.

If you want to see super-cooled water, put some distilled water into a clean plastic ice tray in a freezer set at 26-28 degrees F. Make sure that the sections are not quite full so that if one freezes it will not initiate freezing of the others. Check it after one night. Remove it carefully so as not to disturb. You will probably see that while some of the sections are completely frozen, others are still liquid. Now touch a piece of ice to one of the non-frozen units. You will see it turn to slush in about one second. (It will not freeze completely because of the latent heat released as it freezes.) As it does so it will return to 32 degrees.

I'd still like to know why or what would cause it to be raining even though it's 28f. Last Friday in N. Auburn I went to work in the morning in 28f weather with moderate rain. I came home to 27f weather in moderate rain. It was not freezing rain, just regular. Shouldn't it have been freezing rain? Or snow?

I noticed that this evening with just one inch of snow that traffic in the Portland area rivals that of Seattle for that little bit of snow:http://www.opb.org/news/article/oregon-snow-students-stranded-schools-buses/

Is it because that is snow on top of ice?

I knew that freezing rain and ice storms are a big thing in Portland, and along the Gorge. The one really big issue is that the ice coating can get thick enough to cause tree branches to break and injure people. I remember when I lived in Minnesota* for a year during 7th grade that one morning at the school bus stop I was admiring how beautiful the trees were when they were covered in a layer of ice (a half inch!). Another kid told me that to stay away from the trees because they can break and hurt you (which is why the lovely icicles on houses are also no fun).

By the way, a not so severe version happens in Seattle where the snow will fall at about freezing, turn to water on the slightly warmer pavement and just a bit later becomes black ice. Sometimes it gets covered in snow, and sometimes you just don't see it. You will still slide around.

* As an Army brat I moved around lots, though mostly in places like Oklahoma, California, South Carolina, Texas, and even further south. As a kid I wondered what it would be like to have an actual winter. That year in MN cured me of that notion forever. Now my youngest is getting to know it since she moved to Madison, WI for graduate school.